1. Field of the Invention
The invention relates to the monitoring of the integrity of position and speed information obtained from a hybridization between an inertial unit and a satellite positioning receiver. It more specifically relates to a navigation device known in the art by the name of INS/GNSS (standing for “Inertial Navigation System” and “Global Navigation Satellite System”) that is hybridized in closed loop mode, the hybridization being said to be loose because it is implemented on geographic lines.
2. Description of the Related Art
An inertial unit is made up of a set of inertial sensors (gyrometric sensors and accelerometric sensors) associated with electronic processing circuitry. A computation platform, called virtual platform PFV, then delivers the speed and position information of the bearer in a precise frame of reference (denoted LGT, or local geographic trihedron). The virtual platform PFV is used to project and integrate the data obtained from the inertial sensors. The inertial unit supplies information that is accurate in the short term but tends to drift in the long term (under the influence of sensor defects). Control of the sensor defects represents a very high proportion of the cost of the inertial unit.
A satellite positioning receiver supplies position and speed information concerning the bearer by triangulation based on the positions of orbiting satellites visible to the bearer. The information supplied can be temporarily unavailable because the receiver needs to have in direct view a minimum of four satellites of the positioning system to be able to establish a point. It is also of variable accuracy, dependent on the geometry of the constellation on which the triangulation is based, and affected by noise because of the reliance on the reception of very weak signals originating from distant satellites having a low transmit power. However, they are not subject to drift in the long term, the positions of the orbiting satellites being known accurately in the long term. The noises and errors can be linked to the satellite systems, to the receiver or to the propagation of the signal between the satellite transmitter and the GNSS signal receiver. Furthermore, the satellite data may be erroneous as a consequence of failures affecting the satellites. This non-integrated data must then be identified to prevent it from falsifying the position obtained from the GNSS receiver.
To anticipate the satellite failures and ensure the integrity of the GNSS measurements, a known method is to equip a satellite positioning receiver with a so-called RAIM (Receiver Autonomous Integrity Monitoring) system (an accuracy and availability estimation system) which is based on the geometry of the satellite constellation used in the triangulation and on the predictable short-term trend of this geometry deduced from the knowledge of the trajectories of the satellites. However, this system, purely linked to the satellite location system, is not applicable to the monitoring of a location point derived from a hybrid system (INS/GNSS) and can detect only certain types of failures in a given time.
The hybridization consists in mathematically combining the position and speed information supplied by the inertial unit and the satellite positioning receiver to obtain position and speed information by exploiting both systems. Thus, the accuracy of the positioning information supplied by the GNSS system can be used to control the inertial drift and the inertial measurements that are little affected by noise can be used to filter the noise on the measurements of the GNSS receiver. This combination very often makes use of the Kalman filtering technique.
Kalman filtering is based on the possibilities of modeling the trend (or evolution) of the state of a physical system considered in its environment, by means of a so-called “evolution” equation (a priori estimation), and of modeling the dependency relationship existing between the states of the physical system concerned and the measurements by which it is perceived from outside, by means of a so-called “observation” equation to allow for a readjustment of the states of the filter (a posteriori estimation). In a Kalman filter, the actual measurement or “measurement vector” is used to produce an a posteriori estimate of the state of the system which is optimal in the sense that it minimizes the covariance of the error made on this estimation. The estimator part of the filter generates a priori estimates of the state vector of the system using the deviation observed between the actual measurement vector and its a priori prediction to generate a corrective term, called innovation. This innovation is applied to the a posteriori estimate of the state vector of the system and results in the optimal a posteriori estimate being obtained.
In the case of a hybrid INS/GNSS system, the Kalman filter receives the position and speed points supplied by the inertial unit and the satellite positioning receiver, models the trend of the errors of the inertial unit and delivers the a posteriori estimate of these errors which is used to correct the positioning and speed point of the inertial unit.
The estimation of the position and speed errors due to the defects of the inertial sensors appearing at the output of the virtual platform PFV of the inertial unit is produced by the Kalman filter. The correction of the errors through the intermediary of their estimation done by the Kalman filter can then be done at the input of the virtual platform PFV (closed loop architecture) or at the output (open loop architecture).
When the defects of the sensors of the inertial unit are not too great, there is no need to apply the corrections at the input of the virtual platform PFV. The modeling of the system (linearization of the equations governing the evolution of the system) within the filter remains valid. The a posteriori estimate of the errors of the inertial unit calculated in the Kalman filter is used only to create the best estimate of the position and speed of the bearer given the position and speed information supplied by the inertial unit and by the GNSS receiver. The hybridization is then said to be “open loop”.
When the inertial defects are too great, the linearization of the equations governing the evolution of the inertial model integrated within the Kalman filter is no longer valid. It is therefore essential to apply the corrections to the virtual platform PFV to remain within the linear domain. The a posteriori estimate of the errors the inertial unit calculated in the Kalman filter is used not only to create the best estimate of the position and speed of the bearer, but also to readjust the inertial unit within the virtual platform PFV. The hybridization is then said to be “closed loop”.
The hybridization can also be done by observing GNSS information of different types. Either, the position and speed of the bearer resolved by the GNSS receiver can be considered, in which case the hybridization is said to be “loose”, or the information extracted upstream by the GNSS receiver—the pseudo-distances and pseudo-speeds (quantities directly derived from the measurement of the propagation time and from the Doppler effect of the signals sent by the satellites to the receiver)—can be considered, in which case the hybridization is said to be “tight”.
With a closed loop INS/GNSS system in which the point resolved by the GNSS receiver is used to readjust the information originating from the inertial unit, it is necessary to pay particular attention to the defects affecting the information supplied by the satellites, because the receiver that receives the information will propagate these defects to the inertial unit, resulting in an incorrect readjustment of the latter. The problem becomes particularly critical when it comes to ensuring the integrity of a hybrid INS/GPS point.
A known way of proceeding to monitor the integrity of an INS/GNSS hybrid system in closed loop mode is disclosed in U.S. Pat. No. 5,583,774. It consists in spacing the readjustments by a time that is long enough (for example 30 minutes) for a Kalman-filter-based detector monitoring the trend of the pseudo-distance and pseudo-speed measurement deviations, relative to the bearer of each visible satellite, to be able to isolate the malfunctioning satellites.
Another known method for monitoring the integrity of an INS/GNSS hybrid system is disclosed in U.S. Pat. No. 5,923,286. It involves the use of an RAIM device to enable or disable the hybridization. When the RAIM device signals a loss of integrity, the hybridization is frozen and the position and speed point is supplied by the INS unit taking into account its drifts and bias measured just before the loss of integrity. For this to work, it is essential for the inertial unit not to have been corrupted by the point error committed by the GNSS receiver, which prohibits that being readjusted with the GNSS receiver. The method is therefore reserved only for the open loop INS/GNSS hybrid systems.